Duct liner is a layer of insulation installed on the inside surface of sheet metal HVAC ducts. Its primary jobs are reducing noise from the air handling system, preventing energy loss as conditioned air travels through the ductwork, and controlling condensation. Most duct liner is made from fiberglass bonded with thermosetting resin, though closed-cell elastomeric foam is a growing alternative. Unlike duct wrap, which goes on the outside of the duct, liner sits directly in the airflow path, giving it a unique advantage for sound absorption but also requiring specific material standards for fire safety, air quality, and durability.
How Duct Liner Reduces Noise
Sound control is the main reason specifiers choose duct liner over external insulation. Air handling equipment, the rush of air through ducts, and even conversations traveling between rooms all generate noise that duct liner absorbs. No external insulation product, including elastomeric foams and reflective wraps, matches the acoustical effectiveness of fiberglass liner applied inside the duct.
Acoustic performance is measured by a Noise Reduction Coefficient (NRC), where higher numbers mean better sound absorption. A 1-inch fiberglass or acoustic liner typically carries an NRC around 0.75, meaning it absorbs roughly 75% of the sound energy hitting its surface. Stepping up to a 2-inch liner pushes the NRC above 1.0, which represents near-complete absorption across the mid and high frequency range. This makes thicker liners especially useful in schools, hospitals, recording studios, and office buildings where quiet airflow matters.
Thermal Performance and R-Values
Duct liner also prevents conditioned air from losing heat (or gaining it, in summer) as it moves through unconditioned spaces like attics, crawlspaces, and between floors. Thermal performance is rated in R-value, with higher numbers indicating better insulation. A standard 1-inch fiberglass liner provides roughly R-4.2 when tested flat. At 2 inches, the nominal rating climbs to around R-8.
There’s an important catch: once the insulation wraps around a cylindrical duct, the actual R-value drops below the flat-tested number. A liner rated at R-8 when flat delivers closer to R-5 on a small 4-inch diameter duct and about R-6 on a 20-inch duct. The curve of the insulation compresses the inner surface and stretches the outer surface, reducing effective thickness. Larger ducts see less of this effect because the curve is gentler. If you’re comparing insulation options for small-diameter ductwork, keep this real-world reduction in mind.
Fiberglass vs. Elastomeric Foam
Fiberglass is the traditional duct liner material and still dominates commercial installations. It’s made from glass fibers bonded together, creating a rigid but lightweight board that resists dimensional changes over time. The surface facing the airstream is treated to minimize friction and make it cleanable. Fiberglass liners comply with ASTM C 612, the standard specification for mineral fiber board insulation.
Elastomeric foam liners, typically made from an NBR/PVC blend, are a closed-cell alternative. Because the cells are sealed, foam resists moisture absorption better than fiberglass. Some products in this category are free of formaldehyde and fibers entirely, and carry UL GREENGUARD Gold certification for low volatile organic compound (VOC) emissions. Foam liners also incorporate antimicrobial agents during manufacturing to resist mold and bacterial growth. They handle both thermal insulation and sound absorption, though fiberglass generally outperforms foam on pure acoustics.
Fire and Smoke Safety Requirements
Because duct liner sits inside the airflow path, fire codes hold it to strict limits. NFPA 90A and 90B require that duct linings have a maximum flame spread index of 25 with no evidence of continued progressive combustion, and a maximum smoke developed index of 50. These numbers come from standardized fire testing (ASTM E84 or UL 723). The same limits apply to any tapes, mastics, or sealants used to secure the liner that are exposed to air inside the system. Any product labeled for interior duct use will meet these thresholds, but it’s worth verifying the rating if you’re sourcing materials independently.
Air Velocity Limits
Air rushing past the liner surface can erode it over time, so manufacturers set maximum velocity ratings. Fiberglass duct liners are designed for systems operating at up to 6,000 feet per minute (fpm), with a service temperature range between 40°F and 250°F. When air velocities exceed 4,000 fpm, metal nosing must be applied to the upstream edges of each lined section. These small metal strips protect the leading edge of the liner from peeling or fraying where airflow hits it first. In most residential systems, air velocity stays well below 4,000 fpm, so erosion is rarely a concern. High-velocity commercial systems need more attention to edge protection.
Mold, Moisture, and Indoor Air Quality
Duct liner’s position inside the airstream means any mold growth on its surface can release spores directly into occupied spaces. Research testing eight brands of fiberglass duct liners, including three with built-in biocides, found that antimicrobial treatments helped inhibit fungal growth under high humidity as long as the liner surface stayed clean and dry. When liners became wet or contaminated with dust and debris, even biocide-treated products failed to prevent mold.
This makes moisture control the most important factor for long-term air quality. Properly sized HVAC systems that prevent condensation inside ducts, combined with good sealing at joints, keep liner surfaces dry. If fiberglass liner does become wet or visibly moldy, both the EPA and the National Air Duct Cleaners Association (NADCA) recommend replacing the affected sections rather than trying to clean or treat them. No biocides are currently EPA-registered for use on fiberglass duct liner or duct board.
Duct Liner vs. Duct Wrap
Duct wrap is insulation applied to the outside of the duct, and it serves a different set of priorities. Wrap excels at preventing condensation on the exterior surface of cold ducts running through warm, humid spaces. It also protects the duct itself from physical damage. However, because it sits outside the airstream, wrap does very little to absorb noise traveling inside the ductwork.
If acoustical performance is the primary goal, liner is the better choice. If the main concern is condensation dripping off cold ducts in an attic or mechanical room, external wrap addresses that directly. In some installations, both are used together: liner inside for noise control and thermal performance, wrap outside for condensation protection. The decision often comes down to the building type. Schools, medical facilities, and offices where occupant comfort depends on quiet airflow lean toward liner. Residential systems in humid climates frequently rely on wrap or pre-insulated flexible duct.
Cleaning and Maintenance
Lined ductwork requires more care during cleaning than bare sheet metal. The NIH and NADCA guidelines emphasize avoiding aggressive mechanical cleaning methods that could tear or dislodge the liner surface. Chemical biocides and sealants should not be applied inside lined ducts, as even EPA-registered products can cause eye, nose, and skin irritation when introduced to the airstream. The safest approach is gentle vacuuming with HEPA-filtered equipment and contact cleaning that doesn’t abrade the liner’s surface coating. If liner is damaged, saturated, or supporting visible mold growth, replacement of that section is the standard recommendation rather than attempted remediation.